1. Field of the Invention
The present invention generally relates to techniques for forming enriched surface layers in substrate surfaces. More particularly, this invention relates to a method of enriching a substrate surface with atoms of a solute dissolved in the substrate, so as to form a thin enriched surface layer containing a uniform dispersion of the solute without requiring a supplemental deposition step.
2. Description of the Prior Art
There are various applications in which it is desirable to enrich a surface of a substrate with a material different from the bulk substrate material. The intent is typically to achieve properties at the surface of the substrate that differ from the bulk substrate material, or to otherwise modify the overall properties of the substrate. One such application is for aluminum conductors (lines) typically employed as metal interconnects in semiconductor microcircuits. Aluminum lines are subject to deterioration from electromigration, which is generally the movement of atoms caused by the interaction between electrons and ions in the presence of an electric current. Enriching the grain boundaries and external interfaces of an aluminum line with copper can improve the electromigration resistance of the line by retarding aluminum diffusion. In the past, copper enrichment of aluminum lines has typically been accomplished by forming CuAl2 precipitates at the grain boundaries during deposition by physical vapor deposition (PVD), such as sputtering from an aluminum target containing a small amount (e.g., up to about 2 atomic percent) of copper. However, by their very nature precipitates are laterally heterogeneous, resulting in copper-depleted zones in the otherwise enriched external surfaces, internal grains and boundaries. Consequently, the degree to which electromigration resistance of an aluminum line can be improved by conventional copper enrichment techniques is significantly limited.
From the above, it can be seen that it would be desirable to provide a method for enriching a substrate surface in a manner the yields a more uniform dispersion of the enrichment material. A particular application and advantage would be the ability to produce an aluminum line with a uniform, near-homogeneous dispersion of elemental copper in its surface to sharply limit the surface diffusion of aluminum in the line, thereby improving the electromigration resistance of the line.
According to the present inventions there is provided a method of enriching the surface of a substrate with a solute material that was originally dissolved in the substrate material, to yield a uniform dispersion of the solute material at the substrate surface. The method generally entails producing the substrate to contain a solvent material in which the solute material is dissolved, and which is more reactive than the solute material to a reactive agent. The surface of the substrate is then reacted with the reactive agent to form a reaction compound of the solvent material. Because the solvent material is more reactive than the solute material, the reaction compound of the solvent material forms at a rate greater than any potential reaction compound of the solute material. The reaction process produces a layer of the reaction compound of the solvent material at the surface of the substrate. As the compound layer develops, the solute material segregates or diffuses out of the compound layer and into the underlying substrate, such that the underlying region of the substrate nearest the compound layer becomes enriched with the solute material. At least a portion of the compound layer is then removed without removing the underlying enriched region of the substrate.
In view of the above, those skilled in the art will appreciate that the method described above can be employed to enrich the surface of an aluminum line with elemental copper. Because aluminum has a much higher free energy of oxide formation than copper, the surface of an aluminum line containing a limited amount of copper can be oxidized using a suitable oxidizing agent to produce an aluminum oxide (alumina) layer on the surface of the line. By properly controlling the oxidation rate to occur at a rate sufficiently slow to allow elemental copper to diffuse from the alumina layer into the underlying substrate, the region of the substrate immediately beneath the alumina layer is enriched with a uniform dispersion of the elemental copper from the alumina layer. Thereafter or simultaneously, the alumina layer can be thinned without disturbing the copper-enriched region so as to promote the desired effects of the copper enrichment while avoiding any detrimental effect of excess alumina at the line surface. The resulting aluminum line is characterized by a uniform, near-homogeneous dispersion of elemental copper in its surface that can significantly improve the electromigration resistance of the line.
Other objects and advantages of this invention will be better appreciated from the following detailed description.